Method and apparatus for operating a combustion chamber for autoignition of a fuel

ABSTRACT

A method and apparatus for operating a combustion chamber for autoignition of a fuel includes spraying fuel into a hot gas through a plurality of fuel lances disposed in a mixing zone of a combustion chamber for autoignition of the fuel. The fuel lances are supplied with fuel as two individually supplied groups. Fuel is supplied first to a first group of lances and increased to reach an intermediate operating temperature of about 1100° C. The fuel supply to the second group is then activated, and the fuel supply increased to reach the same intermediate operating temperature with the second group. After both groups are operating at the intermediate temperature, the fuel supply to both groups is simultaneously increased to reach the operating temperature of the combustion chamber.

BACKGROUND OF THE INVENTION

The present invention relates to a method for operating a combustionchamber for autoignition of a fuel. It also relates to a combustionchamber for carrying out the method.

DISCUSSION OF BACKGROUND

In burner configurations with a premixing zone and with an outlet opento the downstream combustion space in the flow-off direction, theproblem repeatedly arises of how a stable flamefront, along withextremely low emission values, can be brought about in the simplestpossible way. Various inherently unsatisfactory proposals have alreadybecome known in this regard. On exception which has become knownhitherto is the invention which is disclosed in U.S. Pat. No. 4,932,861to Keller et al and in which the proposals regarding flamestabilization, efficiency and emissions of harmful substances constitutea leap forward in terms of quality.

A typical firing plant, in which said techniques are bound to fail inthe face of a flame flashback, relates to a combustion chamber designedfor auto-ignition. This is, as a rule, an essentially cylindrical tubeor an annular combustion chamber, into which a working gas flows at arelatively high temperature from approximately 850° C., and there theformation of an auto-igniting mixture is initiated by means of asprayed-in fuel. The caloric treatment of the working gas into hot gastakes place solely within this tube or this annular combustion chamber.If it is a postcombustion chamber taking effect between a high-pressureand a low-pressure turbine, then it is impossible, if only for reasonsof space, to install a premixing zone or premixing burner and to provideor install aids against a flame flashback, which is why this inherentlyattractive combustion technique has hitherto had to be relinquished. Ifthe assumption is to provide an annular combustion chamber as apostcombustion chamber of a gas-turbine group mounted on a single shaft,then additional problems arise with regard to minimizing the length ofthis combustion chamber which are related to flame stabilization.However, even if the solution for flame stabilization is satisfactory,the initial yield of various emissions of harmful substances is stillnot yet solved. The critical range is between the autoignition act and atemperature of approximately 1100° C. In this range, high emissions,particularly in the form of CO and UHC, are produced, and these nolonger comply with the legislation of many countries. Only when thecombustion temperature is higher than 1100° C. is it possible to have agood burn-up along with minimized emissions of harmful substances.

SUMMARY OF THE INVENTION

The invention is intended to remedy this. The object on which theinvention, as defined in the claims, is based is, in a method and acombustion chamber of the type initially mentioned, to minimizeparticularly the CO and UHC emissions in the critical range betweenauto-ignition and a temperature of approximately 1100° C.

It is proposed to lower the emissions of harmful substances by puttingthe existing burners into operation in steps. For this purpose, theburners are to be divided into at least two groups. The individualgroups are run up successively in series from the auto-ignition point toat least 1100° C.

The essential advantage of the invention is to be seen in that, as aresult of the serial run-up into a subcritical domain, the load rangecharacterized by high emission values of harmful substances,particularly with regard to CO and UHC emission values (UHC=unsaturatedhydrocarbons), can be significantly bridged. The group of burners whichis used is supplied, on average, with a larger quantity of fuel duringthe starting phase; the individual burners can thus be operated morestably. When all the burner groups have been brought up to a temperaturestep of approximately 1100° C. they are subsequently run up from thistemperature step in parallel to the desired operating temperature.

Advantageous and expedient developments of the set object according tothe invention are defined in the further dependent claims.

An exemplary embodiment of the invention is explained in more detailbelow by means of the drawings. All elements not necessary for animmediate understanding of the invention are omitted. Like elements areprovided with the same reference symbols in the various figures. Thedirection of flow of the media is indicated by arrows.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 shows an auto-igniting combustion chamber designed as an annularcombustion chamber,

FIG. 2 shows a diagrammatically recorded stepped starting phase in thecase of an autoigniting combustion chamber, and

FIG. 3 shows a qualitative recording of the emission values for harmfulsubstances between a non-stepped and a stepped operating mode during thestarting phase in the case of an auto-igniting combustion chamber.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views, in FIG. 1there is, as emerges from the shaft axis 16, an annular combustionchamber 1 which has essentially the form of a continuous annular orquasi-annular cylinder. Furthermore, such a combustion chamber can alsoconsist of a number of axially, quasi-axially or helically arranged andindividually self-contained combustion spaces. Such annular combustionchambers are preeminently suitable to be operated as auto-ignitingcombustion chambers which are placed in the direction of flow betweentwo turbines mounted on a shaft. When such an annular combustion chamber1 is operated by auto-ignition, the turbine 2 acting upstream isdesigned only for a part expansion of the hot gases 3, as a result ofwhich the exhaust gases 4 downstream of this turbine 2 still flow at avery high temperature into the inflow zone 5 of the annular combustionchamber 1. This inflow zone 5 is equipped on the inside and in thecircumferential direction of the channel wall 6 with a row ofvortex-generating elements 100, referred to below only as vortexgenerators. The exhaust gases 4 are swirled by the vortex generators 100in such a way that, in the downstream premixing zone 7, no recirculationareas occur in the wake of said vortex generators 100.

Arranged in the circumferential direction of this premixing zone 7designed as a Venturi channel are a plurality of fuel lances 8 whichtake over the supply of a fuel 9 and of a supporting air 10. These fuellances 8 will be discussed in more detail further below. The supply ofthese media to the individual fuel lances 8 can be carried out, forexample, via a ring conduit (not shown). The swirl flow initiated by thevortex generators 100 ensures a large-volume distribution of the fuel 9introduced, at best also of the admixed supporting air 10. Furthermore,the swirl flow ensures a homogenization of the mixture of combustion airand fuel. The fuel 9 sprayed into the exhaust gases 4 by the fuel lance8 initiates auto-ignition, insofar as these exhaust gases 4 have thatspecific temperature which can initiate the fuel-dependentauto-ignition. If the annular combustion chamber 1 is operated with agaseous fuel, a temperature of the exhaust gases 4 from approximately850° C. must be present for the initiation of auto-ignition. As alreadyacknowledged above, with such combustion, there is the inherent risk ofa flame flashback. This problem is overcome on the one hand by designingthe premixing zone 7 as a Venturi channel and on the other hand byarranging the spray-in of the fuel 9 in the region of the greatestcontraction within the premixing zone 7. As a result of the narrowing inthe premixing zone 7, the turbulence is reduced because the axialvelocity is increased, the risk of flashback being minimized on accountof the reduction in the turbulent flame velocity. On the other hand, thelarge-volume distribution of the fuel 9 continues to be guaranteed,since the circumferential component of the swirl flow originating fromthe vortex generators 100 is not impaired. The premixing zone 7, whichis kept relatively short, is followed downstream by a combustion zone11. The transition between the two zones is formed by a radialcross-sectional jump 12 which initially induces the throughflow crosssection of the combustion zone 11. A flamefront is also established inthe plane of the cross-sectional jump 12. In order to prevent the flamefrom flashing back into the interior of the premixing zone 7, theflamefront must be kept stable. For this purpose, the vortex generators100 are designed so that no recirculation yet takes place in thepremixing zone 7; only after the sudden cross-sectional widening is itdesirable for the swirl flow to burst open. The swirl flow assists therapid repositioning of the flow behind the cross-sectional jump 12, sothat a high burn-up, along with a short overall length, can be achievedbecause the volume of the combustion zone 11 is utilized as fully aspossible. During operation, there forms within this cross-sectional jump12 a flow boundary zone, in which the negative pressure prevailing theregives rise to the shedding of vortices which then leads to astabilization of the flamefront. The exhaust gases 4 treated in thecombustion zone 11 to form hot gases 14 subsequently load a furtherturbine 14 acting downstream. The exhaust gases 15 can subsequently beused to operate a steam circuit, in the last-mentioned case the plantthen being a combination plant.

FIG. 2 shows a diagram, in which the stepped operating mode of theburners during the starting phase is evident. The abscissa 17 isintended to symbolize the layout of the burners arranged next to oneanother, whilst the ordinate 18 shows the first temperature stepsapproached during the starting phase. In the stepped operating mode, theburners, that is to say the fuel lances of FIG. 1, are supplied seriallywith fuel during the starting phase. In a first step 19, the fuel lances8a, 8c, etc. are put into operation and are first brought toapproximately 1100° C. Subsequently, in a two-step operating mode, theremaining fuel lances 8b, 8d, etc. are likewise brought to saidtemperature level of approximately 1100° C. As soon as all the burnershave been brought to this new temperature step 20, they are then run upjointly, that is to say in parallel, to the desiredoperating-temperature step 21. Since the burners put into C) operationin steps are operated in each case with a larger quantity of fuel, it ispossible to run through the range having high emission values with aricher mixture, as already mentioned above, with the result that theburners can initially be operated more stably. However, this operatingmode has the additional advantage that particularly the CO and UHCemissions can be lowered significantly in the critical range between1000° C. and 1100° C. The stepped operating mode during the startingphase is not restricted to 2 groups of burners.

FIG. 3 shows a qualitative comparison relating to the emissions ofharmful substances between a non-stepped and a stepped operating mode.In the diagram, the abscissa 22 shows the load range, zero being thattemperature level at which the auto-ignition of the mixture takes place,that is to say, in this case, from approximately 850° C. The ordinate 23shows the degree of emissions of harmful substances. The curve 24 showsthe trend of the emissions of harmful substances in the case of aconventional non-stepped operating mode. The peak symbolizes the CO andUHC yield in the interval between approximately 1000° C. andapproximately 1100° C. The stepped operating mode is different, as shownby the curve 25. A two-hump trend, corresponding to the steppedoperating mode with two burner groups, can be seen here. In terms oforder of magnitude, with the stepped operating mode, emissions which areless than half those of the conventional operating mode can be achieved.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that, within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed:
 1. A method for operating a combustion chamber,comprising the steps of:guiding a hot gas having a temperaturesufficient to initiate auto-ignition of a fuel into a combustion space,spraying a fuel into the hot gas via a plurality of fuel lances, whereinthe fuel lances are connected for fuel supply in at least two separatelycontrollable groups, wherein during start up of the combustion chamber,spraying the fuel comprises the steps of:supplying fuel first to a firstgroup and increasing the fuel to reach a temperature of about 1100° C.,supplying fuel in addition to the first group to a second group andincreasing the fuel to reach a temperature of about 1100° C., and whenthe at least two groups are operating at about 1100° C., increasing thefuel to all the fuel lances to reach an operating temperature.
 2. Themethod as claimed in claim 1, wherein the fuel lances are supplied withfuel and supporting air.
 3. A combustion chamber for autoignition of afuel, comprising:an inflow zone to receive a main flow of hot gas, theinflow zone enclosed by a peripheral wall; a plurality of vortexgenerators mounted on the peripheral wall of the inflow zone to createvortices in the main flow; a premixing passage connected to receive themain flow from the inflow zone; a plurality of fuel lances disposed forspraying fuel into the premixing zone; and a combustion zone bounded bya wall and connected to receive the main flow with injected fuel fromthe premixing passage, wherein the wall of the combustion zoneconnecting to an outlet of the premixing passage has a radiallyextending portion forming a cross-section expanding jump.
 4. Thecombustion chamber as claimed in claim 3, further comprising means forsupplying fuel to the fuel lances as at least two individually suppliedgroups.
 5. The combustion chamber as claimed in claim 3, wherein thepremixing passage is a Venturi-shaped channel.